This invention relates to improved methods and apparatus for processing workpieces, more particularly, processing workpieces for electronic device fabrication.
The successful processing of materials for electronic devices typically requires optimization and precise control of the processing environment at all process steps. Many of these process steps are performed under conditions that make it difficult or impossible to measure the desired process variables. In those cases where an important process variable cannot be readily measured (e.g. semiconductor wafer temperature during a plasma etch step for an integrated circuit), an attempt is made to correlate the parameter of interest to other measurable or controllable parameters. The accuracy and stability of these correlations, also called equipment response models, are a critical factor in determining the process capability and device yield at any given process step.
An accurate equipment response model for a given process step will typically have both spatial (dependence upon location within the process tool) and temporal (dependence upon process time or sequence) components. In applications such as semiconductor wafer processing, the industry trend toward larger wafers makes it increasingly important that not only the average values but also the distribution and uniformity of critical process parameters be measured. This spatial mapping requirement usually requires distribution of multiple sensors within the processing area; consequently, there may be increased probability of process perturbation and reduced response model quality. Obtaining process time dependent data typically requires in-situ and substantially real time instrumentation and measurement. These measurement techniques often have requirements (e.g. optical access, electrical connections, etc.) that are incompatible with or intrusive to the processing environment and tool configuration.
Obtaining or verifying an equipment response model under these conditions can be difficult, expensive, and problematic. The introduction of potentially perturbing sensor elements and their associated feedthroughs and connections into the process environment (e.g. thermocouples into a plasma discharge) is undertaken with great reluctance. Therefore, most of the intrusive applications are only used by equipment developers and, to a lesser extent, high-end process developers. Even though such applications will clearly benefit high volume production, advance instrumentation and modeling is almost never used in such environments. This is because the danger of process perturbation usually exceeds the potential benefits.
Equipment response models are often highly sensitive to specific process parameters, sometimes in ways that are not readily apparent. For example, in a typical plasma etch system the wafer temperature can be dependent upon the process gas mix and wafer backside roughness as well as the more obvious parameters of chuck temperature, RF power, and backside helium pressure. Equipment response models developed with an incomplete understanding of all interactions or that are made under conditions significantly different from the actual manufacturing conditions can have serious errors. It is unlikely that one could adequately anticipate the final optimum processing conditions and wafer states so as to produce a generally acceptable response model; this is particularly true during the design and development stage when measurements for response models are typically made.
Equipment response models can be highly sensitive to hardware variations such as surface finish on an electrostatic chuck used in semiconductor wafer processing. An attempt to stabilize a response model by specifying tight tolerances on numerous attributes of a component is usually costly and ineffective. Important hardware attributes often undergo a slow change over time, and these changes may be reflected as drift in the form of slowly increasing inaccuracy in the equipment response model.
Clearly, there are numerous applications requiring reliable and efficient methods and apparatus by which spatially resolved and time resolved equipment response models can be easily and economically developed and maintained. An example of an important application is the uniform processing of workpieces such as semiconductor wafers, flatpanel displays, and other electronic devices. Furthermore, there is a need for methods and apparatus capable of collecting data for response models in a nonperturbing manner on unmodified process equipment running realistic process conditions. Still further, there is a need for methods and apparatus capable of generating, checking, and frequently updating response models for individual pieces of equipment in a manufacturing facility so as to improve the operating efficiency, improve the productivity, and reduce the cost of ownership for the equipment and for the overall manufacturing facility.